Operating circuits in devices and semiconductor chips draw current from a supply network. These supply networks are typically inductive in nature and as such they provide a constant supply voltage during stabilized operations if the current dissipated by the circuits is constant. As such, there is minimal self-induced electrical noise (voltage fluctuation) generated by the supply network while supplying constant current. However, the inductive supply network rejects any immediate changes in current so that immediate changes in supplied current required as a result of changes in current dissipation of the circuits are rejected by the supply network. Instead, the supply network requires some period of time to adjust and stabilize the supply voltage at the newly required current level.
The changes in current required for supply to operating circuits causes a voltage disturbance among other local circuits of the device because, during the adjustment period, any difference between the requited and supplied currents has to be accounted for by the local circuits. Specifically, capacitance of the local circuits is acting as storage for electrical energy, providing the changes in current until such time as the supply network stabilizes at the new current supply level. The changes in capacitance include increased/decreased capacitance charges, for example. These changes of electrical energy stored in the capacitance lead to voltage/current distortions in the local circuits that lead to generation of self-induced electrical noise, also referred to as supply noise.
A specific type of circuit commonly found in processing systems is a differential driver circuit or differential driver. The differential driver generally includes two single-ended drivers connected to a differential bus. One of the single-ended drivers couples data on a signal line of the bus while the other single-ended driver couples a complement of the data on another signal line of the bus. The current dissipated by a single-ended driver typically depends on the logic value of the data transmitted by the driver. Since differential drivers simultaneously transmit the data and its complement, the differential drivers always transmit low and high logic values during every data transmission regardless of the actual data value. The simultaneous transmission of low and high logic values means the differential drivers dissipate a constant current when active, a situation that results in minimal self-induced supply noise during the transmission periods.
Turning to a bidirectional bus system used for example in high-speed time-shared memory systems, the same bus lines are used to transmit data in two directions between circuits. Therefore, when data is transmitted from component A to component B, a differential driver at component A transmits data while a receiver at component B receives the data. Likewise, when data is transmitted from component B to component A, a differential driver at component B transmits data while a receiver at component A receives the data. The process of reversing direction of the data transfer is referred to as bus turnaround.
In performing bus turnaround, generally, the transmitter at component A and the receiver at component B are deactivated, and a differential driver at component B then transmits data to a receiver at component A. The process of activating/deactivating differential drivers during bus turnaround typically causes voltage/current changes in the local circuits, and these local voltage/current changes interrupt the current dissipation in the driver circuits thereby generating supply noise. This supply noise is a major source of simultaneous switching noise (SSN), jitter, and signal distortion introduced into transferred signals by the system.
The supply noise generated during bus turnaround might be reduced by allowing the drivers of a component to always remain active and disconnecting the driver from the bus during periods when the component is receiving data. Alternatively, the driver output can be directed onto another signal path during periods when the component is receiving data. However, disconnection or redirection of the driver output requires control circuits and accurate timing synchronization of the disconnection or redirection processes, where the timing involves coordinating between both the transmitting and the receiving components. Any perturbations in this timing would produce brief intervals where both drivers (transmitting component and receiving component) were connected to the bus or both drivers were disconnected from the bus, thereby resulting in local current changes and the associated supply noise.
The local current perturbations and hence the supply noise related to bus turnaround might also be reduced by keeping the drivers of all components active and superimposing signals of transmitting components and receiving components on the bus. The receivers would then require circuitry/logic for subtracting the data transmitted by a driver of the receiving component from the received data for periods during which that component is receiving data. While this solution avoids most timing issues it requires the use of additional circuitry/logic for processing the superimposed signals at the receivers. Further, this solution does not eliminate the need to exercise separate control over the driver currents in both the transmitting and receiving components.
The processing speed of electronic systems continues to increase and, as such, so does the need for high-speed transfer of information among components of these systems. As the accurate high-speed transfer of information depends partly upon the quality of the signaling channel, a need remains to reduce/eliminate noise in the signaling channel of high-speed processing systems.
In the drawings, the same reference numbers identify identical or substantially similar elements or acts. To easily identify the discussion of any particular element or act, the most significant digit or digits in a reference number refer to the Figure number in which that element is first introduced (e.g., element 102 is first introduced and discussed with respect to FIG. 1).